This invention relates generally to devices employed in the testing of local area network (LAN) cables and in particular to an instrument which tests the relevant parameters of a copper-conductor LAN cable by measuring the capacitance between all pairs of conductors and employing a software method operating on the stored measurement data.
Local area networks (LAN's) now connect a vast number of personal computers, workstations, printers, and file servers in the modem office. A LAN system is most commonly implemented by physically connecting all of these devices with copper-conductor twisted-pair LAN cables, the most common being an 8-wire cable which is configured in 4 twisted-wire pairs with each end of the cable terminated in an industry-standard connector. Some LAN cables include a flexible foil wrapper that acts as an electrostatic shield. In a typical installation, LAN cables may be routed through walls, floors, and ceilings of the building. LAN cable systems require constant maintenance, upgrades, and troubleshooting because LAN cables and connectors are subject to breakage, offices and equipment must be moved, and new equipment are added.
The tasks of installing, replacing, or re-routing cables typically fall on a professional cable installer or in-house network maintenance person. During the installation phase, each cable is routed through the building and a connector is attached to the each end of the new cable. Each wire in the cable must be connected to its proper respective electrical connection at both ends of the LAN cable in order for the LAN connection to function properly. A variety of LAN cables are used in the industry, including: unshielded twisted pair ("UTP"), shielded twisted pair ("STP"), and coaxial cables. LAN cable installation practices, cable performance specifications, and building wiring practices are governed by the Electronic Industries Association Commercial Building Telecommunications Wiring Standard EIA/TIA 568.
Such connections can be tested with an electrical resistance-measuring instrument commonly known as an ohmmeter which tests the direct current (d.c.) resistance through the electrical path between the ohmmeter's test leads. Using the ohmmeter to effectively test a LAN cable requires detailed knowledge of the proper connections. The end of the LAN cable system in which the test instrument is applied is the "near-end". The other end of the LAN cable thereby becomes the "far-end". With a known termination such as resistors at the far-end of the cable to provide a complete circuit, the "wire map" or set of connections can be discerned, along with short-circuit and open-circuit wiring errors. This manual technique of probing connections quickly becomes prohibitively inefficient and time-consuming. For a cable of N wires, a total of N!/(N-2)! measurements must be performed for a complete test between all pairs of the N wires, probing each respective pair with both negative and positive polarity.
Specialized LAN cable test instruments have been developed to diagnose the most commonly encountered cable problems. The instrument automatically performs a series of resistance measurements thereby relieving the operator of the burden of probing individual connections manually. The instrument performs continuity checks on the cable to ensure that all the connections exist as required by industry standard definitions and provides the operator with a visual indication of continuity and proper connection of each wire pair through the cable. Tests for open-circuit errors, short-circuit errors, and crossed pair errors are provided.
The least expensive LAN cable test instruments are essentially specialized ohmmeters equipped to test industry-standard terminations and wire maps according to EIA/TIA-568. Such instruments allow for direct connection to the LAN cable, along with a special terminating device ("cable identifier"), which is connected to the far end of the cable to facilitate a known return path for the d.c. test current provided by the instrument for each specified wire pair through resistors with pre-determined values and diodes with pre-determined polarities. Such instruments suffer from several marked disadvantages however. First, because the instrument is limited to d.c. measurements, the set of tests that the instrument can perform is limited, leaving many of the potential faults in a cable undetected. A critical parameter is the amount of coupling between wire pairs, commonly referred to as cross-talk. Each of the four wire pairs is twisted together within the cable and that respective pairing must be maintained in order to get proper isolation from the other pairs. A common wiring error is to cross a wire pair at one end of the cable while duplicating the error at the other end. While the connection appears proper according to the d.c. measurement, the wire pairs are no longer twisted separately but are now commingled, resulting in an unacceptable level of cross-talk between the two wire pairs.
The second disadvantage of cable mapping instruments is that a cable identifier must be connected to the far-end of the cable being tested to provide a known return path for the d.c. test current. When the far-end of the cable is hundreds of feet away, the process of testing the cable becomes cumbersome, often requiring two people, one at the near-end and one at the far-end, to perform the task of troubleshooting and verifying multiple cables, often long after the initial installation has been completed, resulting in costly re-work.
More sophisticated LAN cable test instruments are often equipped for evaluating cross-talk between wire pairs at the near-end of cable through standardized near-end cross talk (commonly referred to as "NEXT") measurements. NEXT is a measure of the level of isolation between separate wire pairs. A NEXT test is typically performed by injecting a high frequency alternating current (a.c.) test signal into a wire pair at the near-end of the cable, often at frequencies similar to actual data rates which range as high as 16 MHz, measuring the signal level induced in each of the other pairs as measured at the near-end, and comparing the induced signal level with the injected signal level to determine the level of isolation. A higher level of isolation between wire pairs is necessary to avoid interference between data communication paths. The consequence of inadequate isolation between wire pairs is degraded communications reliability and increased error rates. Inadequate NEXT isolation is a symptom of a number of possible problems including incorrectly wired LAN connectors or telephone-grade cables that do not meet the specifications for data communications. The disadvantage of an instrument providing only a NEXT reading is that the instrument may not provide any additional information regarding the source of the problem, leaving the operator to troubleshoot the problem.
Accordingly, an instrument that can provide troubleshooting and verification of the proper connection of a variety LAN cable systems with a more complete set of automated diagnostic tests without the necessity of applying a cable identifier at the far-end of the LAN cable would be desirable.